Pkg-config file use difference Cflags convention to ogg/vorbis/flac

[erikd]

Other XIph libraries:

> grep ^Cflags /usr/lib/x86_64-linux-gnu/pkgconfig/ogg.pc 
Cflags: -I${includedir}

> grep ^Cflags /usr/lib/x86_64-linux-gnu/pkgconfig/vorbis.pc 
Cflags: -I${includedir}

> grep ^Cflags /usr/lib/x86_64-linux-gnu/pkgconfig/flac.pc 
Cflags: -I${includedir}

Opus:

> grep ^Cflags /usr/lib/x86_64-linux-gnu/pkgconfig/opus.pc 
Cflags: -I${includedir}/opus

Ogg/Vorbis/Flac then encourage the relevant header files to be included as:

#include <ogg/header.h>
#include <vorbis/header.h>
#include <FLAC/header.h>

which mitigates the possibility of header file names from one project clashing with another.

Please consider doing the same for Opus.